Cancer is /not/ mold. Cancer is the plastic of a pony breaking down and does not spread. Mold is a bunch of little specks embedded into the plastic in an area. It looks like a bunch of blackhead zits or something. It's often called pin-head mold, which is very descriptive as to what it looks like. Mold can spread, but if you clean the pony well, it won't. Bleach, boiling, and the like can kill it, but may also promote the disintegration of the plastic (and thus cause cancer eventually, though I think all ponies are going to succumb to that eventually).